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Lithium deintercalation of Li x CoO 2 from x ) 1 to x ≈ 0 has been carried out electrochemically. The changes in the electronic structure from LiCoO 2 to CoO 2 have been investigated by X-ray photoelectron spectroscopy (XPS) to bring some new developments about the electron transfer mechanisms upon lithium deintercalation. All available XPS core peaks (Co 2p, Co 3p, Co 3s, O 1s, F 1s, P 2p, C 1s) and valence spectra have been analyzed. The contributions of the electrode material and of the electrode/electrolyte interface have been clearly distinguished. We show that cobalt and oxygen simultaneously undergo a partial oxidation process and that the sole participation of oxygen atoms to the charge transfer process, as it is sometimes assumed, can be excluded. The surface film consists of organic and inorganic species resulting from degradation of the electrolyte.

Materials prepared by chemical Li deintercalation with NO2BF4 from Li1.20Mn0.54Co0.13Ni0.13O2 and chemical Li reinsertion with LiI show very similar chemical composition, oxidation state of each transition metal ion, structural properties and electrochemical performance to those of the material recovered after the 1st electrochemical cycle. Investigations combining redox titration, magnetic measurement, neutron diffraction and chemical analyzes reveal that uncommon redox processes are involved during the first charge at high voltage and explain the charge overcapacity and large reversible discharge capacity obtained for this material. This further assesses our proposal that oxygen, in addition to nickel and cobalt, participates to the redox processes in charge: within the bulk oxygen is oxidized without oxygen loss, whereas at the surface oxygen is oxidized to O2 and irreversibly lost from the structure. During the subsequent discharge, in addition to nickel, cobalt and oxygen, manganese is also slightly involved in the redox processes (reduction) to compensate for the initial surface oxygen loss.

The 6,7 Li MAS NMR spectra of lithium ions in paramagnetic host materials are extremely sensitive to number and nature of the paramagnetic cations in the Li local environments and large shifts ͑Fermi contact shifts͒ are often observed. The work presented in this paper aims to provide a rational basis for the interpretation of the 6,7 Li NMR shifts, as a function of the lithium local environment and electronic configuration of the transition metal ions. We focus on the layered rocksalts often found for LiMO 2 compounds and on materials that are isostructural with the K 2 NiF 4 structure. In order to understand the spin-density transfer mechanism from the transition metal ion to the lithium nucleus, which gives rise to the hyperfine shifts observed by NMR, we have performed density functional theory ͑DFT͒ calculations in the generalized gradient approximation. For each compound, we calculate the spin densities values on the transition metal, oxygen and lithium ions and map the spin density in the M-O-Li plane. Predictions of the calculations are in good agreement with several experimental results. We show that DFT calculations are a useful tool with which to interpret the observed paramagnetic shifts in layered oxides and to understand the major spin-density transfer processes. This information should help us to predict the magnitudes and signs of the Li hyperfine shifts for different Li local environments and t 2g vs e g electrons in other compounds.

Li y (Ni 0.425 Mn 0.425 Co 0.15 ) 0.88 O 2 materials were synthesized by a slow rate electrochemical deintercalation from Li 1.12 (Ni 0.425 Mn 0.425 Co 0.15 ) 0.88 O 2 during the first charge and the first discharge in order to study the structural modifications occurring during the first cycle and especially during the irreversible "plateau" observed in charge at 4.5 V vs Li + /Li. Chemical Li titrations showed that the lithium ions are actually deintercalated from the material during the entire first charge process, excluding the possibility that electrolyte decomposition causes the "plateau". Redox titrations revealed that the average transition metal oxidation state is almost constant during the "plateau", despite further lithium ion deintercalation. 1 H MAS NMR data showed that no Li + /H + exchange was associated to the "plateau" itself. Rietveld refinement of the XRD pattern for a material reintercalated after being deintercalated at the end of the "plateau", as well as redox titrations, revealed an M/O ratio larger than that of the pristine material, which is consistent with the oxygen loss proposed by Dahn and coauthors for the LiNi x Li (1/3-2x/3) Mn (2/3-x/3) O 2 materials to explain the irreversible overcapacity phenomenon observed upon overcharge. X-ray and electron diffraction showed that the transition metal ordering initially present within the slabs is lost during the "plateau" due to a cation redistribution. To explain this behavior a cation migration to the vacancies formed by the lithium deintercalation from the transition metal sites (3a) is assumed, leading to a material densification.

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